Migrate BuildSystem.prepare to a BSP request

Sources/BuildSystemIntegration/BuildServerBuildSystem.swift

@@ -329,10 +329,7 @@ extension BuildServerBuildSystem: BuiltInBuildSystem {
     return nil
   }
 
-  package func prepare(
-    targets: [BuildTargetIdentifier],
-    logMessageToIndexLog: @Sendable (_ taskID: IndexTaskID, _ message: String) -> Void
-  ) async throws {
+  package func prepare(request: PrepareTargetsRequest) async throws -> VoidResponse {
     throw PrepareNotSupportedError()
   }